1. Field of Invention
This invention relates to homogenous catalyst formulations for methanol production from synthesis gas and methods of producing methanol by using the catalyst formulations of the invention.
2. Description of the Related Art
The transport of natural gas from remote sites such as deposits embedded in the permafrost or methane hydrates beneath the ocean floor continues to present a technological challenge. The transport of compressed natural gas under pressure such as or of liquefied natural gas via liquefaction offer limited applications for the transportation of natural gas located at remote sites. As a result, the conversion of natural gas into transportable liquid fuel at the well-head is an attractive option. Under the gas-to-liquid option, natural gas is first converted into synthesis gas, which is primarily a mixture of CO, CO2, H2. The synthesis gas can subsequently be processed catalytically into either hydrocarbons via Fischer-Tropsch technology or methanol. Both the Fisher-Tropsch technology and methanol synthesis are commercially practiced but the carbon monoxide conversion efficiency remains an issue. Therefore, there is still a need for an improved yet economical gas-to-liquid process for use in facilitating the transportation of natural gas from remote sites.
Methanol can be manufactured by catalytic hydrogenation of carbon monoxide as shown in reaction [1]:                                           CO            +                          2              ⁢                                                           ⁢                              H                2                                              ⇔                                    CH              3                        ⁢            OH            ⁢                                                   ⁢            Δ            ⁢                                                   ⁢            H                          =                              -            128.6                    ⁢                                           ⁢          k          ⁢                                           ⁢                      J            .                          mol                              -                1                                                                        [        1        ]            
The operating temperature controls the CO conversion in this highly exothermic, reaction. Conventional commercial heterogeneous catalysts operate at approximately 250° C. and high pressure of about 750 psi and are limited by low equilibrium conversion that thermodynamically allows less than 20% conversion per pass of CO.
U.S. Pat. Nos. 4,935,395 and 4,992,480 incorporated herein by reference, describe two component catalyst systems consisting of tetracarbonyl nickel Ni(CO)4 and alkali metal alkoxide of alkali metals such as Li, Na, K, Cs. More specifically, these patents disclose a catalyst system consisting of Ni(CO)4 and potassium methoxide dissolved in a methanol solvent and a co-solvent such as tetrahydrofuran or p-dioxane. The catalyst system operates preferably between 100°-150° C. At these mild temperatures, per pass conversion of greater than 90% is achieved to yield methanol from synthesis gas with a H2/CO ratio of 2/1.
However, the system disclosed in these patents has two significant limitations. First, Ni(CO)4, must be handled with a great deal of caution because it is quite toxic, having an OSHA limit of 1 ppb. Secondly, impurities in the synthesis gas (“syngas”) such as CO2 and H2O interact with the alkoxide base slowly leading to the deactivation of the catalyst system. As a result, the removal of CO2 from the synthesis gas prior to it entering the methanol synthesis reactor renders the catalyst systems described above economically unattractive.
The cause of the deactivation of the catalyst is primarily due to interaction of the alkoxide base with CO2 and H2O via reactions [2] and [3] respectively:KOCH3+CO2→KOCO2CH3  [2]KOCH3+H2O→KOH+CH3OH  [3]
Alternately, synthesis gas entering the methanol synthesis reactor must be purified to remove these impurities to ppm level thus adding to the process cost. Reversing Reactions [2] and [3] will yield active KOCH3, a process that facilitates methanol synthesis.
It is also recognized that the undesirable reaction of KOCH3 with CO2 to yield potassium methyl carbonate (KOCO2CH3 or PMC) as shown in Reaction 2 is exothermic and readily proceeds at room temperature. The equilibrium constant (Keq) of this reaction is reported to be 2×108 M−1 at 0° C. for sodium methoxide (C. Faurholt. Z. Physik. Chem. 126, pp. 72, 85, 211, 227 (1927)). This reaction ties up KOCH3 that is needed to activate CO thereby disrupting the Ni-catalyzed methanol synthesis cycle when synthesis gas containing CO, CO2, H2 is used directly without prior CO2 removal. Furthermore, CO2 can be subsequently processed via the water-gas-shift (“WGS”) reaction to yield CO for conversion to methanol.
 H2+CO2→CO+H2O  [4]